This invention relates to a peaking control circuit which functions automatically to reduce the high frequency response of a video amplifier when weak television signals are being received.
The high frequency components of a composite television video signal correspond to the smallest picture elements in a horizontal line. If the high frequency components are not adequately amplified, the rapid changes between dark and light colors for small adjacent picture elements in a horizontal line cannot be reproduced on the television screen. This produces a picture with reduced horizontal detail, commonly referred to as a "soft" picture. Adequate amplification of the high frequency components, on the other hand, produces a picture with greater horizontal detail, commonly referred to as a "crisp" picture.
When the high frequency response of the video amplifier produces a crisp picture, any high frequency noise in the received signal is emphasized together with the information-carrying high frequency components. When a strong signal is being received, the high frequency amplification necessary for producing a crisp picture is low enough so that any high frequency noise will not have an adverse effect on the picture. However, when the received signal is weak, such that a substantial portion of its high frequency content is noise, the video amplification necessary to produce a crisp picture may be great enough so that the amplified high frequency noise produces a substantial deterioriation in picture quality. Thus, it is not always desirable to provide the video amplifier with the high frequency response needed for a crisp picture.
Because of this noise problem, some video amplifiers have included means for manually altering their high frequency responses. Such peaking control circuits enable an operator to manually adjust for a very crisp or a very soft picture in accordance with high frequency signal-to-noise conditions or viewer preference. One known peaking control circuit utilizes a variable resistor in series with a capacitor in the emitter circuit of a video amplifier transistor. When the resistor is adjusted to its minimum resistance value, minimum degeneration and thus maximum peak frequency response is obtained. Another manual peaking control circuit of this type is shown in U.S. Pat. No. 2,627,022, in which manual reduction of peaking also results in reduction of the AGC threshold level.
Peaking control circuits which require manual operation are not satisfactory. User satisfaction depends upon the operator of the receiver having some appreciation of the intended functions of the controls and their effect on the television picture. A more important factor contributing to user dissatisfaction is the necessity of manually adjusting the peaking control each time there is a change in received signal strength, which may occur each time the receiver is tuned to a different channel.